1. Field of the Invention
The present invention relates to an injection molding machine including a controller having a function for preventing damages to the screw by monitoring and controlling screw rotating torques.
2. Description of the Related Art
A known injection molding machine has a function for protecting a screw by limiting or stopping screw operation immediately after an excessive screw rotating torque exceeding a preset allowable upper limit of screw rotating torque is sensed.
The allowable upper limit of screw rotating torque to be set is determined by calculating the dynamic strength such as tensile strength and torsional fatigue strength of the screw material, or by actually measuring torques in a test in which metering is tentatively performed under conditions in which the screw rotating torque is sufficiently limited.
However, there are cases in which screws break below their respective allowable upper limits of screw rotating torque, so there is a need for an improved injection molding machine that enables an allowable upper limit of screw rotating torque to be set more properly. Since the amount of torque that breaks the screw is far greater than the torque normally used for molding, the screw rotation speed should be limited by a torque closer to the level actually used for molding. If the screw rotation speed is not limited when the screw rotating torque abnormally increases due to any cause, the screw will be subject to a heavy load up to the allowable upper limit.
In a known method, as disclosed in Japanese Patent Application Laid-Open No. 7-32430, screw breakage is prevented by setting an allowable upper limit of screw rotating torque. In a known method for setting an allowable upper limit of screw rotating torque, the dynamic strength of the material is calculated to determine the allowable upper limit. For precise calculation, however, it is required to collect many data of the strength of the screw material such as tensile strength and torsional fatigue strength. In addition, the screw is used in high temperature environment and the number of continuous molding cycles for production exceeds the number of continuous molding cycles for a normal fatigue test. Furthermore, screws widely vary in shape. Accordingly, it is difficult to determine precisely the strength of a screw simply by calculating the dynamic strength of the material.
In another known method for setting an allowable upper limit of screw rotating torque, a reference screw rotating torque required for metering operations is determined from the values actually measured during tentative metering operations and an allowable upper limit of screw rotating torque that will not adversely affect molding is set. If not automated, however, this method is time-consuming because metering conditions vary with resin types and the articles to be molded.
In another known method, as disclosed in Japanese Patent Application Laid-Open No. 6-297532, a discriminating section and a tolerable range are set depending on the distance the screw moves and the time elapsed from the start of metering, and a driving torque of the screw rotating motor in the discriminating section is detected and compared with the tolerable range to detect abnormal metering. This technique requires an operator to determine and manually input the tolerable range of driving torque of the screw rotating motor. It is difficult to determine the tolerable range, which widely varies with molds and resin types. Even if automatically set, this tolerable range is only used to monitor the driving torque of the screw rotating motor and determine the quality of the molded articles, not for preventing screw breakage.
As disclosed in Japanese Patent Application Laid-Open No. 9-174637, there is a known function that sets a limit torque value of the electric servomotor for continuous molding operation on the basis of the actually measured torques with which good kneading and plasticization of the resin are accomplished in trial shots. The limit torque is set with a sufficient margin for the trial shots and the torques of the electric servomotor are actually measured during the trial shots. Here, the torques actually measured along the distance the screw moves in the entire metering process are plotted, but it is not necessary to plot all the actually measured torques to set a limit torque value. Only essential data should be selected for measurement and stored to reduce storage capacity demand.